Measurements of Reproductive Traits in a Field Population of Meadow Voles

Abstract

Neonatal litter size, pup growth, and birth interval were examined within a population of meadow voles ( Microtus pennsylvanicus ) through use of radiotelemetry and live trapping. Litter sizes observed shortly after birth were significantly smaller than predicted from embryo counts obtained during the same period. The observed litter size was correlated negatively with local population density. Examination of the birth interval indicated ≥95% of females conceived during a postpartum estrus. Pup growth rates ( X = 0.44 g/day) did not vary significantly with litter size, maternal weight, or population density. We postulate that selection for an increase in litter frequency limits the range of growth rates possible for individuals in a litter, and that resource reduction at high densities results in a reduction in litter size postpartum, rather than reductions in pup growth rates, litter frequency, or prepartum litter size.